This invention relates generally to electrophotographic printing, and more particularly, a cleaning blade used therein to remove particles adhering to the photoconductive member.
In the process of electrophotographic printing, a photoconductive surface is charged to a substantially uniform potential. The photoconductive surface is imagewise exposed to record an electrostatic latent image corresponding to the informational areas of an original document being reproduced. This records an electrostatic latent image on the photoconductive surface corresponding to the informational areas contained within the original document. Thereafter, a developer material is transported into contact with the electrostatic latent image. Toner particles are attracted from the carrier granules of the developer material onto the latent image. The resultant toner powder image is then transferred from the photoconductive surface to a sheet of support material and permanently affixed thereto.
This process is well known and useful for light lens copying from an original and printing applications from electronically generated or stored originals, and in ionography.
In a reproduction process of the type as described above, it is inevitable that some residual toner will remain on the photoconductive surface after the toner image has been transferred to the sheet of support material (e.g. paper). It has been found that with such a process that the forces holding some of the toner particles to the imaging surface are stronger than the transfer forces and, therefore, some of the particles remain on the surface after transfer of the toner image. In addition to the residual toner, other particles, such as paper debris (i.e. Kaolin, fibers, clay), additives and plastic, are left behind on the surface after image transfer. (Hereinafter, the term "residual particles" encompasses residual toner and other residual particles remaining after image transfer.) The residual particles adhere firmly to the surface and must be removed prior to the next printing cycle to avoid its interfering with recording a new latent image thereon.
Various methods and apparatus may be used for removing residual particles from the photoconductive imaging surface. Hereinbefore, a cleaning brush, a cleaning web and a cleaning blade have been used. Both cleaning brushes and cleaning webs operate by wiping the surface so as to affect transfer of the residual particles from the imaging surface thereon. After prolonged usage, however, both of these types of cleaning devices become contaminated with toner and must be replaced. This requires discarding the dirty cleaning devices. In high-speed machines this practice has proven not only to be wasteful but also expensive.
The shortcomings of the brush and web made way for another now prevalent form of cleaning known and disclosed in the art of blade cleaning. Blade cleaning involves a blade, normally made of a rubberlike material (e.g. polyurethane) which is dragged or wiped across the surface to remove the residual particles from the surface. Blade cleaning is a highly desirable method, compared to other methods, for removing residual particles due to its simple, inexpensive structure. However, there are certain deficiencies in blade cleaning, which are primarily a result of the frictional sealing contact that must occur between the blade and the surface.
Dynamic friction is the force that resists relative motion between two bodies that come into contact with each other while having separate motion. This friction between the blade edge and the surface causes wearing away of the blade edge, and damages the blade's contact with the surface. For purposes of this application, volume wear (W) is proportional to the load (F) multiplied by the distance (D) traveled. Thus, W .varies.FD .varies.FVT, or introducing a factor of proportionality K, W=KFVT where K is the wear factor, V is the velocity and T is the elapsed time. Hence, wear increases with larger values of K. Various blade lubricating materials or toner lubricant additives have been proposed to reduce friction which would thereby reduce wear. However, lubricants tend to change the operational characteristics of the printing machine undesirably. For example, a polyurethane blade with a good lubricant in the toner can ideally achieve a frictional coefficient of about 0.5, however, this rarely occurs because of the delicate balance involved in achieving the proper weight percent of lubricant in the toner. Normal frictional coefficient values for cleaning blades that remove toner off the imaging surface range from a low of about 0.5 to a high of about 1.5.
In addition to the problem of wear, which is more or less predictable over time, blades are also subject to unpredictable failures. The impact from carrier beads remaining on the charge retentive surface subsequent to development may damage the blade, and sudden localized increases in friction between the blade and surface may cause the phenomenon of tucking, where the blade cleaning edge becomes tucked underneath the blade, losing the frictional sealing relationship required for blade cleaning. Additionally, slight damage to the contacting edge of the blade appears to eventually initiate tearing sites. These problems require removal and replacement of the blade. It is an objective of the present invention to provide a cleaning blade member which exhibits improved blade tip tear resistance.
Investigation into the characteristic of cleaning blade performance has shown that lateral conformance of the blade, i.e., conformance of the blade across the imaging surface, is generally given by EQU .epsilon..varies.1/E
where
.epsilon. is blade conformance in microns; PA1 E is the Young's modulus for a given elastomer. PA1 .omega..sub.o is the resonant frequency of the blade.
A high value for lateral conformance is very desirable, and accordingly, for a given blade, Young's modulus should be small.
It has also been determined that for the blade to optimally respond to roughness in the imaging surface, particularly at high speeds, the resonant frequency of the blade must be as high as possible. Resonant frequency of a blade is given by EQU .omega..sub.o .varies..sqroot.E
where
A high resonant frequency for optimal frequency response is very desirable, and accordingly, for a given blade, Young's modulus for the selected elastomer should be large.
It can be seen that the use of isotropic materials, such as the urethane cleaning blades currently used in electrophotographic cleaning processes, requires a trade off in the selection of materials having a Young's modulus that satisfactorily meets both the lateral conformability requirement, and the resonant frequency requirements.
The commonly used elastomer-type cleaning blade is a resilient material that allows stubborn residual particles to remain on the surface. This occurs because the resilient elastomeric material is unable to provide sufficient contact to create a tight seal between the cleaning blade and the surface when tuck occurs, therefore the resiliency of the elastomeric blade makes it easy for the blade to glide over the residual particles.
One approach to increase cleaning blade life is to improve the blade wear rate. The physical and geometrical changes observed in a blade due to wear is believed to be one of the key elements that causes the blade to lose its cleaning efficiency. Since this poor service wear life can lead to frequent blade replacements, it is therefore, very costly and taxes our customer service system as well. It is an object of this invention to improve the wear life and durability thus reducing blade replacements.
While it might appear that a rigid metal blade might solve the problems of rigidity and wear, in fact, the frictional contact required between the surface and blade quickly wears away the blade and any surface lubricants applied thereto. As the blade edge wears, it changes from a chiseling edge to a rounded or flattened surface which requires a high force to maintain the edge in sealing contact. While a beveled edge is useful in liquid toner applications, it is highly susceptible to damage and wear in dry toner applications. Accordingly, it is desirable to maintain the blade's square edge without wear. Additionally, wearing friction may generate toner fusing temperatures, causing toner to fuse to the blade, or the surface. Furthermore, filming on the surface can deteriorate image quality. Filming occurs either uniformly or as streaking, due to deficiencies in blade cleaning, requiring the use of a lubricant and a balancing abrasion element to prevent filming. It is an object of the present invention to reduce the frictional contact between a cleaning blade and an imaging surface.
Further objectives of the present invention include: providing a cleaning blade member with improved resistance to fatigue cracking; and providing a cleaning blade member having substantial mechanical stability and extended service life.
Various cleaning techniques have hereinbefore been used as illustrated by the following disclosures, which may be relevant to certain aspects of the present invention:
"Impregnated Poromeric Material Cleaning Blade," Xerox Disclosure Journal, et al., Vol. 1, No. 4, April 1976, p. 79 describes a cleaning blade composition of non-woven polyester fibers bound together in polyurethane, for the improvement of abrasion resistance, hardness, resilience, and load bearing capacity.
"Nylon Fiber Reinforcement for Polyurethane Composites," Polymer Composites, Cordova et al., Vol. 8, No. 4, August 1987, pp..253-255, suggest polyurethane thermoset material with a nylon fiber filler for improved impact strength, impact fatigue and decreased stress cracking.
U.S. Pat. No. 2,767,529 to Scott describes a doctor blade for paper making machines made of metal or layers of fabric bonded together by synthetic resin.
U.S. Pat. No. 3,635,556 to Levy suggest a backing pad made of a carbon filled plastic foam material.
U.S. Pat. No. 3,915,735 to Moreland describes a monomeric silane sprayed or poured onto microcrystalline novaculite while it is being agitated in a high intensity mixing apparatus at a temperature between 70.degree. F. and 350.degree. F., and the monomeric silane and microcrystalline novaculite are allowed to remain in situ at a temperature between about 70.degree. F. and 350.degree. F. for at least about 1 minute.
U.S. Pat. No. 4,549,933 to Judd et al. describes a composite doctor blade with nonhomogeneous stiffness properties and having a plurality of juxtaposed fibrous layers which are encapsulated in an epoxy resin. The composite blade has a fibrous core, intermediate uni-directional graphite layers and outer fibrous layers. The uni-directional graphite fibers in the intermediate layers are oriented in the machine direction.
U.S. Pat. No. 4,823,161 to Yamada et al. describes a cleaning blade design which has a double-layer structure comprising a contact member first layer made of a poly(urethane) ureamide polymer, held in contact with a toner image bearing member surface, and a supporting member second layer adhered to the contact member first layer to provide improved blade function. The support member for the contact member has the same hardness or essentially the same hardness as the contact member and is lower than the contact member in glass transition temperature.
U.S. Pat. No. 4,825,249 to Oki et al. describes a cleaning blade for a photoelectronic copy machine comprising a substrate of urethane rubber and a coating of perflouropolyether.